The present invention relates, in general terms, to torsion-damping devices of the type comprising at least two coaxial parts mounted rotatably relative to one another within the limits of a specific angular movement and elastic means interposed circumferentially between the said parts.
As is known, such a torsion-damping device is conventionally used to form a clutch plate assembly, especially for a motor vehicle, one of its rotary parts then comprising a friction disc intended to be fixed in rotation to a first shaft, in practice a drive shaft, the output shaft of the engine, for example, a motor vehicle, while another of the the rotary parts is carried by a hub intended to be fixed in rotation to a second shaft, in practice a driven shaft, for example the input shaft of a gear box in the particular case of such a motor vehicle.
Such a torsion-damping device makes it possible to ensure controlled transmission of the torque applied to one of its rotary parts when the other is itself subjected to a torque, that is to say to absorb the vibrations liable to occur over the entire length of the kinematic chain which then extends from the engine to the controlled shafts of the wheel on which it is inserted.
In practice, the elastic means interposed circumferentially between the two rotary parts in question, conventionally called elastic means with circumferential action, are distributed in several "stages" of different rigidities, which intervene as a function of the angular movement between the rotary parts. At the start of this angular movement, only a stage of relatively low rigidity is involved, and, in proportion to the development of the angular movement, one or more other stages of relatively higher rigidity act in turn, then adding their effects to those of the first.
Thus, the value of the total rigidity of the active elastic means with circumferential action can be matched appropriately to the value of the torque to be transmitted.
Likewise in practice, friction means also act between the rotary parts in question to achieve the desired damping.
As is known, this results in a hysteresis effect that is to say a differentiation, for a given value of angular movement between the rotary parts in question, between the value of the torque transmitted between these rotary parts during an increasing change in the torque and the value of this torque during a decreasing change.
Of course, the friction torque attributed to the friction means used must be matched to the value of the rigidity of the corresponding elastic means with circumferential action. When this is low, it must preferably be relatively low itself.
The friction means used in this way between the two rotary parts in question usually comprises at least one friction washer, and this takes effect by means of one of its transverse faces, such a friction washer being in contact by means of this face with any transverse component belonging to one of the rotary parts, while being fixed in rotation to the other of these rotary parts over at least one range of the angular movement between them.
As regards a torsion-damping device in which one of the rotary parts comprises a hub and the other at least one guide washer, arranged around the hub, it has already been proposed, particularly in French Pat. No. 1,537,961, to achieve the desired friction by making use of a bearing located radially between this hub and this guide washer to prevent the latter from inadvertently shearing the hub.
Such a bearing, which is an annular component, then engages by means of an axial bearing surface with an axial bearing surface of the hub on which it is engaged, while being connected in rotation by engagement means to the associated guide washer.
Now as is known, as regards a clutch plate assembly, one of the rotary parts in question is centered on the drive shaft, while the other is centered on the driven shaft.
A misalignment can consequently arise between them, these shafts themselves not necessarily being precisely aligned with one another, particularly since it is becoming increasingly common, at the present time, for the friction disc of a clutch plate assembly for a motor vehicle to be arranged in an overhanging position on the input shaft of the gear box which constitutes the associated driven shaft.
Such a possible misalignment between the two rotary parts in question can result in the friction generated by the bearing, which is used radially between them and which conventionally acts from the start of their angular movement, being too high with regard to the single first stage of relatively low rigidity of the elastic means with circumferential action, which then acts, this being at the risk of nullifying its effects prematurely.
The subject of the present invention is, in general terms, an arrangement making it possible to mitigate this disadvantage and also opening up a new field of use for the relevant friction means.